airfoils

An airfoil is a streamlined shape designed to produce lift when air flows past it. In practice, airfoils are cross-sectional shapes of wings, rotor blades, or blades in hydrofoils, propellers, and wind turbines. A typical cross-section has a curved upper surface and a flatter lower surface, but real wings have finite span, twist, and taper. Key geometric parameters include the chord length (distance from leading to trailing edge), camber (the curvature of the camber line), thickness distribution, and the leading and trailing edges.

Lift arises from differences in pressure created by the airflow as it negotiates the airfoil. When the wing is at an angle of attack, the flow accelerates over the upper surface, lowering pressure, while the lower surface experiences higher pressure. The Kutta-Joukowski theorem provides a theoretical link between circulation around the airfoil and lift; viscous effects in the boundary layer determine drag and stall behavior. At small angles of attack, lift increases with angle; at a critical angle, flow separation causes stall and a rapid loss of lift. The Reynolds number, based on chord length and flow velocity, governs the relative importance of viscous effects.

Airfoils are categorized by camber and thickness. Symmetric airfoils have no camber and generate near-zero lift at zero angle of attack, while cambered airfoils can produce lift with positive lift at zero angle. Designers tailor thickness distribution for strength, weight, and drag. Standard data for many airfoils are published by organizations such as NACA and Eppler, including lift, drag, and moment coefficients across angles of attack and Reynolds numbers. Wind-tunnel testing and computational fluid dynamics are used to evaluate performance.

Applications include aircraft wings, helicopter rotor blades, propellers, hydrofoils, and wind turbine blades. The study of airfoils dates to early aviation pioneers and matured with Prandtl's boundary-layer theory and subsequent development programs.